Unignore signals when starting CloudABI processes.

[ed]

Summary

As CloudABI processes cannot adjust their signal handlers, we need to
make sure that we start up CloudABI processes with consistent signal
masks. Though the POSIx standard signal behavior is all right, we do
need to make sure that we ignore SIGPIPE, as it would otherwise be
hard to interact with pipes and sockets.

Extend execsigs() to iterate over ps_sigignore and call sigdflt() for
each of the ignored signals.

Test Plan

The tests for raise() now pass:
https://github.com/NuxiNL/cloudlibc/blob/master/src/libc/signal/raise_test.c#L55

[kib]

sys/kern/kern_sig.c

982 ↗

(On Diff #7893)

I doubt that any other ABI, besides your ABI, would ever need such hack. IMHO, you should keep the execsigs() interface as is, and test the SV_ABI_CLOUDABI flag instead of the parameter.

Not to mention that the patch become much more compact.

[ed]

Simply test against SV_ABI_CLOUDABI.

sys/kern/kern_sig.c

982 ↗

(On Diff #7893)

That makes sense. Does this version look all right?